This invention relates to an apparatus for and a method of taking a predeterminable volume of a sample of a medium. In particular, the invention deals with an apparatus and a method whereby a sample is taken from a medium flowing in an open channel or flowing by gravity through a pipe.
The medium (frequently one with a high solids content) is commonly drawn from the open channel or pipe by means of a pump over a suction line, generally a hose or tube, and conducted from above into a vertically mounted dosing vessel. The portion of the suction line which extends into the dosing vessel and is commonly referred to as a xe2x80x9cdosing tubexe2x80x9d has on its surface a scale in the form of marks with the aid of which a user can adjust that volume in the dosing vessel which is below the dosing tube in the dosing vessel and corresponds to the selected sample volume.
In a conventional sampling operation, the medium is drawn into the dosing vessel until the rising level of the medium reaches a sensor mounted in the upper part of the dosing vessel, i.e., until the dosing vessel is filled up. The sensor triggers a signal whereby the drawing in of the medium is stopped. Next, the dosing vessel is either pressurized or opened to the atmosphere in order to dose the desired sample volume. The quantity of the volume in which the dosing tube is immersed flows out of the dosing vessel, so that only the desired sample volume remains in the dosing vessel. In addition, the fluid contained in the suction line flows back into the open channel. The sample is then drained from the dosing vessel into one or more sample containers.
In the samplers described, the desired sample volume is adjusted only mechanically via the length of the dosing tube extending into the dosing vessel, namely by changing (generally permanently) the length of the dosing tube which is guided in a lid of the dosing vessel.
In another apparatus, described in CA-A-2,192,149, the length of the dosing tube in the dosing vessel can be changed by rotating the dosing tube, but this method, too, necessitates human intervention.
In conventional apparatus, the desired sample volume can generally be adjusted only when the apparatus is first put into operation or when the sample volume is changed. Changing the volume from one sampling operation to the next as an adaptation to a changing flow rate of the medium in the channel, as is desired, for example, for taking a sample volume proportional to flow rate, is not possible during continuous operation.
Another problem encountered with the sampling systems explained is that initially, more medium is drawn into the dosing vessel than is needed for the sample proper. As the dosing vessel is being filled, the solids of solids-containing media begin to sediment and accumulate in the lower portion of the dosing vessel, from which the sample is ultimately drained into a sample container.
When the excess medium flows out of the dosing vessel (upward) back into the channel, the sedimented solids of the excess medium, too, will remain in the lower part of the dosing vessel. Due to the sedimentation, however, the concentration of the solids in the sample in the lower part of the dosing vessel has already increased to the point that the sample to be drained off into a sample container is no longer representative of the medium to be examined.
It is therefore an object of the invention to provide a sampling apparatus and a sampling method whereby the desired sample volume can be changed in a simple manner without mechanical intervention even during operation, with any accumulation of solids in the sample contained in the dosing vessel being avoided, so that a sample representative of the medium is obtained.
To attain this object, the invention provides an apparatus for taking a predetermined volume of a sample of a medium flowing in an open channel or by gravity through a pipe, said apparatus comprising:
a control unit;
a pneumatic control gear controlled by the control unit;
a pump switched by the control unit;
a sample collecting vessel for receiving the sample volume;
a drain device;
a hose or tube used as a suction line, through which the sample is conveyed via an inlet into the sample collecting vessel; and
a detection unit incorporated in the suction line which determines the presence of the medium in the suction line.
To attain the above object, the invention further provides a method of taking a predetermined volume of a sample of a medium flowing in an open channel or by gravity through a pipe, comprising the following steps:
A control unit switches a pump, which is pneumatically connected with a sample collecting vessel, and at least one valve in a pneumatic control gear comprising different valves in such a manner that a partial vacuum is created in the sample collecting vessel;
through the partial vacuum, the medium is drawn from the channel through the suction line into the sample collecting vessel;
as soon as a detection unit incorporated in the suction line determines a presence of the medium and signals this to the control unit, a preselectable time corresponding to a desired volume of the sample and monitored by the control unit begins to run;
after expiration of the time, the control unit turns the pump off and switches the pneumatic control gear so that the sample collecting vessel is opened to the atmosphere;
under the action of gravity, the medium still contained in the suction line and not needed for the sample flows back into the channel; and
the sample in the sample collecting vessel is drained off into at least one sample container.
In a preferred embodiment of the apparatus according to the invention, the drain device comprises a flexible hose and a squeezing device for squeezing off the hose.
In another preferred embodiment of the apparatus according to the invention, the detection unit is located as close to the inlet of the sample collecting vessel as possible.
In a further preferred embodiment of the apparatus according to the invention, the detection unit comprises an optical, capacitive, or inductive sensor or a conductivity sensor for determining the presence of the medium as well as a signal generator connected to the sensor and providing a signal indicating the presence of the medium to the control unit.
In a further preferred embodiment of the apparatus according to the invention, the pneumatic control gear is a valve block with a camshaft gear or with solenoid valves.
In still another preferred embodiment of the apparatus according to the invention, the sample collecting vessel has, in the area of its inlet, an emergency switch which stops the supply of the medium to the sample collecting vessel.
In a preferred embodiment of the method according to the invention, the time required to draw the medium into, and collect it in, the sample collecting vessel is determined for at least one predetermined sample volume, so that times associated with different sample volumes can be determined, and the time determined for at least one desired sample volume is entered into and/or stored in the control unit.
In another preferred embodiment of the method according to the invention, a time corresponding to a desired sample volume and stored in the control unit is used to control the pump during the sampling operation.
In a further preferred embodiment of the method according to the invention, prior to a new sampling operation, the suction line is blown by the pump with compressed air.
One particular advantage of the invention is that only a quantity of the medium corresponding to the desired sample volume is conducted into the sample collecting vessel. As a result, the solids content of the sample finally drained off into the sample container is identical with the solids content of the medium in the channel or pipe. An additional step for dosing the sample volume to be drained off is not necessary.
Another particular advantage of the invention is that it allows the desired sample volume to be adjusted without the need to shorten or rotate the dosing tube extending into the sample collecting vessel or without the need for any other mechanical intervention in the sampling apparatus. Simply by changing the time predetermined for the sampling, it is possible to adjust a sample volume proportional to the flow rate of the medium in the channel or pipe. Since, in addition, the time controlling the sampling is managed in the control unit and can be stored there, it is possible to automatically adapt the duration of inflow of the medium into the sample collecting vessel, and thus the sample volume, to a measured flow-rate value of the medium in the channel or pipe.